Purifying light metals



Jan. 16, 1934. H. N. GILBERT PURIFYING LIGHT METALS Original Filed Aug. 24

Paieiied Jan'. 16, 1934 PURIFYING LIGHT METALS Harvey Nicholas Gilberti, Niagara Falls, N. y., ae-

signor, by mesne assignments, to E. I. du Pont de Nemours and Company, a corporation of Delaware .Application Aug'ust 24, 1931, Serial No. 558,859

Renewed April 6, 1933 10 Claims.

This invention relates to a process and apparatus for handling and purifying light molten metals.v

Crude light metals and alloys commonly contain as impurities various oxides, salts or other metals, depending on the method of manufacture.

. Such metals are often puried by melting and nltering to remove the impurities which are infusible at the melting point of the metal. The

- more reactive light metals, such as the` alkali metals, when purified in this way, must be handled in `an inert atmosphere to prevent oxidation and hazards to the workmen.

The methods heretofore employed for filtering molten light metals are essentially batch processes and comprise forcing the metal through a :filter mediumI in the bottom of a closed tank by means of gravity and a compressed, inert gas. When the batch of metal is ltered,

the apparatus must be opened to remove the residual sludge, which contains considerable of the metal, mixed with the infusible impurities. The removal of the sludge involves certain hazards due to spattering and burning of the semimolten mass. Furthermore, during this operacovered with several metallic screens of varying neness in order of size from the largest mesh next to the pipe to the smallest mesh on the outside; these screens are sealed in at the ends of the 65 element with heat resistant cement. The element 12 isconnected by means of .pipe 13 to the top of the receiving tank 14. Tank 14 is covered with insulating material 15y and is provided with a valve-litted outlet 16 at the bottom, and at the 7o top with outlet 17, which is connected to a two- Way valve element 18. One side of the valve 18 is connected to a suction pump not shown, while the other side is connected to a gas container.

A large pipe 19, equipped with heating ele- 75 ments 20 and covered with lagging 21, is connected with the bottom of tank land slopes up- .ward at an angle of about45 Pipe'y 19 is tted with screw-conveyor' 22,-which is adapted to be powered at 23 by suitable means. Discharge pipe so tion considerable oxidation of the sludge occursw24 is connected near `the upper end of pipe 19 at which reduces the yield of metal that may be recovered therefrom.

An object of this invention is to provide an eincient processand apparatus for molten metal purification which will give a product of consistently good quality with a minimum of metal x loss. Another object is to provide a continuous process for iiltering molten light metals. A further object is to reduce the hazard in purifying highly reactive light metals. A still further object is to reduce-the amount of labor involved in purifying molten metals. Other objects will appear hereinafter.

These objects are attained', according( to the present invention, by a novel process which com- -prises filtering substantially pure liquid metal in an inert atmosphere from a body of impure liquid metal which is replenished continuously, or intermittently, while the impurities are concomitantly collected and removed. from the body of impure metal. A novel feature of the process is the removal of solids or semi-solids from the surface of the lter medium'by a back-surge of a portion oi.' the puried metal through the lter.

In the drawing, 1 is an iron tank fitted with suitable heating elements'2,cooling nues 3, and lagging 4. Pipe 5 is an inlet for cooling air which. feeds to the flues" 3 around tank 1; 6 is an inlet 55101' inert gas; '7 is an inlet for impure metal.

a point above the intended level of liquid in tank 1. When my inventionis used for obtaining pure sodium from the crude electrolytic metal containing as solid impurities, sodium chloride, sodium oxide, calcium and calcium chloride I may proceed as follows. T he apparatus shown in the drawing is lled with nitrogen passed in through pipe 6, crude liquid metal is fed into tank 1 by way of pipe 7. 'Ihe temperature of the molten 90 metal, as indicated by the thermocouple element 11, is maintained above C. for instance at 10U-120 C. by suitably controllingthe rate of cooling air iiow through pipe 5 and iiues 3 and the heating elements 2. Agitator 10 is operated 95 at abouftXlB R. P. M. and valve 18 is adjusted to partially evacuatetank 14. This partial vacuum, draws substantially pure sodium from the body of metal through immersion lter element 12 and pipe `13 into receiver 14. When the re- 10o ceiver is full or at other times, if plugging of the lter occurs, valve 18 is actuated to shut off the4 f vacuum and simultaneously release inert gas un,- der a slight pressure in 'its stead. The sudden change in pressure in tank 14 causes a momen- 105 tary surge of the column of liquid metal containedyin pipe 134 back into tank l, thus forcing accumulated material from the filter element. During the filtration, the impurities -of greater specic gravity which may be solid or semi-solid, 11()- settle to the bottom of the mildly agitated body of liquid metal. The agitating element 10 partially scrapes the cone-shaped lower sides of tank 1, assisting in that way to collect these solids, which I term sludge, near the conveying means. The conveying element 22 is adapted to carry off these materials either continuously or intermittently. The screw in the conveyor fits loosely in the pipe, so that when operated at a suitable speed, it elevates the sludge outward and upward to the discharge pipe 24 while as much as possible of the liquid metal is allowed todrain back into the tank.

Example 1 Crude sodium as described above was purified by means of an apparatus constructed in accordance with that shown in the drawing. During the process, the level of liquid metal in tank 1 was maintained above the top of the filter unit 12. An inert atmosphere under slight pressure was maintained above themetal by leading in nitrogen by way of pipe 6. The temperature of the molten metal in tank l was maintained at 100-120 during filtration. Agitator 10 was continuously rotated at approximately 18 revolutions per minute and screw element 22, when in use, rotated at about 105 revolutions per minute. The screw element was set at an angleof about 40 from the perpendicular and the screw pitch was 6 inches.

Portions of crudeliquid sodium were introduced through pipe 7. After each addition, suction was applied to the receiver `14 and filtered sodium was drawn therein until the level -.in tank 1 was near the top of iilter unit 12. Valve 18 was then operated to cut off the suction and admit nitrogen into tank 14. The purified sodium was removed from tank 14 by means of the valved pipe 16.

During the filtration, a sludge consisting of solid calcium and salts and oxides of calcium andv sodium settled out in the bottom of tank 1; this sludge, wet with liquid metal, was discharged by way of pipe 24 by intermittent operation of screw element 22.

The purified metal and the sludge from each batch were analyzed; the following' results are the average from 24 consecutive flltrations:

Average content of impurities in filtered sodium, 0.025%.,

The total weight ofthe sludge was about 8.8% of the total weight of the crude sodium introduced into the apparatus.

The sludge contained about 89% of metallic sodium.

The total weight of sodium in the sludge was equivalent to about 6% of the weight of the purined sodium.A Y

The process is most effective when the temperature is so maintained that substantially all impurities are solid and more efficient when the sludge outlet-means is so constructed and sumciently slanted to allow excess liquid metal to drain back to the ltering chamber.

By means of my invention, higher yields may be obtained than was possible with prior methods. 4Iior instance, in the purification of sodium by prior methods, a minimum of around 14% of the metal was removed with the sludge and the impurities'could not be reduced below 0.045%.

With my process only 5-10% of the metal is removed with the sludge and the impiu'ities have been reduced to as low as 0.025%.

. A further important advantage of my method over prior 'methods is that it may be operated continuously or intermittently. It is not necessary to stop operation to add raw metal, clean the filter, or remove sludge.

Since the handling of molten metals, especially the more reactive metals, is attended by considerable hazard, this invention renders a much safer method for purifying such metals and provides a closed, controlled system adapted to carry out the entire process with one apparatus. The metal is continuously maintained under an inert atmosphere, preventing substantially all oxidation. The apparatus may be operated by vacuum rather than pressure and thereby reducing hazards caused by leakage. By the use of my invention, there is a substantial saving in floor space, labor, and oxidation losses.

The removal of sludge from filtration of liquid metal such as sodium has always been diilicult to control with any of the known types of Valves. The sludge tends to' solidify in the valve, preventing complete closure, causing leakage or obstructing the flow. Often the solidified sludge makes operation of the valve so difiicult tha t the operator applies undue force which is Aliable to break the valve or its connections. This constitutes a serious hazard when a considerable body of liquid metal lies above the valve. Myinvention overcomes these disadvantages by providing a substantially valveless system for sludge removal.

Various modifications of my invention may be made without departing from the spirit and scope thereof and although my process is designed to be operated continuously it may be operated intermittently with satisfactory results. It is also possible to employ pressure onl the surface of the body of molten metal in order to assist in iilter- :v ing operations, or even toentirely replace vacuum filtering by pressure ltering. Other types of immersion` lter bodies may' be substituted for that herein described. 'The conveyer may be arranged at varying angles and satisfactory results obtained as long asthe discharge opening is located at some point above the level of the molten metal. Other known'types-of conveyers 120 may be substituted `for the'type shown herein. The process is obviously applicable to various lightmetals and alloys.

By-the term light metals in the specification and claims, I mean all metals or alloys which, at 125 any temperature in the molten state, may contain solid impurities heavier than the liquid metal; I particularly include the alkali and alkaline earth metals. The term inert gas is used to designate any gas which is substantially non- 130 reactive with the metal being refined at the refining temperature.

I claim: I

1. Process for purifying crude light metals comprising maintaining said light metal liquid 185 in an inert atmosphere and at a temperature at which impurities have solidied.. withdrawing light metal from said liquidy through a filter and concomitantly collecting and. removing sludge from said liquid from a level below filtration.

2. Process for purifying crude alkali metal comprising maintainingv said alkali metal liquid in an inert atmosphere and at a temperature at which impurities have solidified, withdrawing alkali metal from said liquid through a yfilter and l i'. concomitantly collecting and removing sludge from said liquid from a level belowltration.

3. Process for purifying crude electrolytic sodium comprising maintaining `said sodium liquid in an inert -atnn'uxphere and at a temperature at 150 which impurities have solidified, withdrawing sodium from said liquid through a lter and concomitantly collecting and removing sludge from said liquid from a level below ltration.

4. Process for purifying crude electrolytic sodium comprising maintaining said sodium liquid at 100-120 C. in an inert atmosphere, withdrawing sodium from said liquid through a metal lterscreen and concomitantly collecting and removing sludge while permitting drainage of molten sodium back into the container from said sludge being removed.

5. Process for purifying crude electrolytic sodium comprising continuously maintaining said sodium liquid at 100-120 C. in an inert atmosphere, mildly agitating, continuously adding crude sodium and continuously withdrawing sodium from said liquid through a metal lter screen, and concomitantly collecting and removing sludge while permitting drainage of molten sodium back into the container 4from said sludge being removed.

6. Process according to claim 2 wherein the flow of alkali metal through the filter is reversed at intervals to clean said lter.

7. Process according to claim 5 where the ow of sodium through the metal lter screen is reversed at intervals in order to clean said screen.

8. Device for purifying crude electrolytic light metal comprising a closed metal chamber provided with heating and cooling means, inlet pipes for crude light metal and inert gas, a iilter means within said chamber, an outlet means extending from said lter means to an outside connected collecting chamber, means for scraping and agitation Within said chamber and below said lter, and means for removing solids from a level below said scraping means.

9. Device for purifying crude electrolytic sodium comprising a closed metal chamber provided with heating and cooling means, inlet pipes for sodium and inert gas, a lter means within said chamber, a pipe extending from said lter means to an outside suction and pressure connected collecting container, scraping and agitating means within said chamber and below said lter, and a conveyor for solids extending from said chamber outward and upward from a level below said scraping means.

10. Device for purifying crude electrolytic s0- dium comprising a closedmetal chamber provided with heatingand cooling means, inlet pipes for sodium and inert gas, a metal lter screen within said chamber, a pipe extending from said lter to an outside suction and pressure connected collecting container, a mechanically driven scraper 'and agitator within said chamber and below said filter, and a motor driven screw conveyor `extending from said chamber outward and upward from a level below said scraper.

HARVEY NICHQLAS GILBERT. 

